1. Field of the Invention
The present invention relates to a motor driver for a brushless motor provided with Hall elements for detecting the rotation position of the rotor, such as a spindle motor used in a CD (compact disk) or DVD (digital versatile disk) drive.
2. Description of the Prior Art
Some conventional brushless motors are of the type that recognizes the rotation position of the rotor by the use of Hall elements and that performs feedback control on the basis of the Hall signals from those Hall elements. As an example of such brushless motors that perform feedback control on the basis of Hall signals, there has been proposed a motor speed controller that recognizes the rotation angle of a motor on the basis of Hall signals and eliminates motor torque ripple components (Japanese Patent Registered No. 3281561). Moreover, as a prior-art technique, there has been proposed a driving controller that, by recognizing the rotor position on the basis of Hall signals, recognizes the conditions of the individual three-phase currents for driving a brushless motor and that varies the duty factors of the individual phases through calculations performed by a microcomputer so as to achieve PWM (pulse-width modulation) control (Japanese Patent Application Laid-Open No. 2001-136772).
In the motor speed controller proposed in Japanese Patent Registered No. 3281561, the rotation angle of the rotor is found on the basis of the magnetic pole position of the rotor as detected by Hall elements and the pulses generated by the motor during a single turn thereof as detected by an MR sensor, then the value to be read out from a torque ripple correction memory is determined on the basis of that rotation angle. Then, the torque ripple component is calculated on the basis of the value read out from the torque ripple correction memory, and this torque ripple component is eliminated from the motor control signal. On the other hand, in the driving controller proposed in Japanese Patent Application Laid-Open No. 2001-136772, the conditions of the individual phases are recognized on the basis of Hall signals, which are square waves, outputted from Hall elements, and the duty factors are set through calculations performed on the basis of the thus recognized conditions and the current levels of the individual phases.
In addition to the configurations proposed in Japanese Patent Registered No. 3281561 and Japanese Patent Application Laid-Open No. 2001-136772, where the Hall signals from Hall elements are used simply to recognize the magnetic position of the rotor or to recognize the conditions of the individual phases, there have conventionally been used also motor drivers of the type that drives and controls a motor by multiplying a torque error signal by Hall signals themselves so as to shift the phases of the individual phases. FIG. 6 shows the configuration of a motor driver that operates on this principle.
The motor driver shown in FIG. 6 includes: an adder circuit 3 that is fed with a torque instruction signal that indicates a target current level; a low-pass filter (LPF) 4 that eliminates noise from the output of the adder circuit 3; a limiter 5 that imposes a limit on the output level of the low-pass filter 4; a position detection circuit 6 that recognizes the magnetic pole position of the rotor by means of Hall elements 2a to 2c of a brushless motor 1 to output position signals, which are sinusoidal waves; multiplier circuits 7a to 7c that multiply the position signals from the position detection circuit 6 by the output level of the limiter 5 to generate pseudo-sinusoidal waves; a phase shift circuit 8 that shifts the pseudo-sinusoidal waves from the multiplier circuits 7a to 7c each by ⅙π to output three-phase pseudo-sinusoidal waves; a PWM conversion circuit 9 that converts the three-phase pseudo-sinusoidal waves from the phase shift circuit 8 individually into PWM signals on the basis of a triangular wave from a triangular wave generation circuit 10; a triangular wave generation circuit 10 that generates a triangular wave; a driving control circuit 11 that outputs, on the basis of the three-phase PWM signals from the PWM conversion circuit 9, driving output currents to be fed to the three-phase coils (not illustrated) provided inside the brushless motor 1; and a current detection circuit 12 that detects the current level of the driving output currents outputted from the driving control circuit 11.
The motor driver shown in FIG. 6 operates in the following manner. In the adder circuit 3, the current level of driving output currents as detected by the current detection circuit 12 is subtracted from a torque instruction signal, which represents the target current level. The resulting torque error signal then has noise eliminated therefrom by the low-pass filter 4, and then has a limit imposed on the level thereof by the limiter 5. On the other hand, when three-phase Hall signals from the Hall elements 2a to 2c, which represent the magnetic pole position of the rotor, are fed to the position detection circuit 6, the position detection circuit 6 generates and outputs position signals with reference to which to feed three-phase driving output currents.
Thereafter, in the multiplier circuits 7a to 7c, the torque error signal from the limiter 5 is multiplied by the position signals, which are sinusoidal waves, to generate pseudo-sinusoidal signals. Then, in the phase shift circuit 8, the phases of the pseudo-sinusoidal signals are shifted each by ⅙π to generate three-phase pseudo-sinusoidal signals. Then, in the PWM conversion circuit 9, these three-phase pseudo-sinusoidal signals are individually subjected to PWM conversion on the basis of the triangular wave outputted from the triangular wave generation circuit 10 to generate three-phase PWM signals. On the basis of these three-phase PWM signals, the driving control circuit 11 generates driving output currents to be fed to the three-phase coil (not illustrated) of the brushless motor 1 to drive and control the brushless motor 1.
With the motor driver configured as shown in FIG. 6, when the amplitude of the Hall signals from the Hall elements 2a to 2c of the brushless motor 1 is within the range that permits normal operation, and the amplitude of the pseudo-sinusoidal signals obtained by multiplying the Hall signals by the torque error signal is not larger than the level corresponding to about twice the PWM conversion pulse width, the motor driver shown in FIG. 6 can feed pseudo-sinusoidal currents to the individual phases. This allows low-vibration, low-noise rotation. Accordingly, in this state, the phase shift circuit 8 outputs pseudo-sinusoidal signals having an amplitude smaller than a predetermined level k as shown in FIG. 7A, and thus the PWM conversion circuit 9 outputs normally converted PWM signals as shown in FIG. 7B.
However, when the amplitude of the Hall signals from the Hall elements 2a to 2c is so large that the amplitude of the pseudo-sinusoidal signals obtained by multiplying the Hall signals by the torque error signal is larger than the level corresponding to about twice the PWM conversion pulse width, distortion occurs in the current level of the driving output currents outputted from the driving control circuit 11. Accordingly, in this state, the phase shift circuit 8 outputs pseudo-sinusoidal signals having an amplitude larger than the predetermined level k as shown in FIG. 7C, and thus, when the pseudo-sinusoidal signals have a level higher than k or lower than −k, the range in which the duty ratio is 100% or 0% is so wide that the resulting PWM signals remain high or low almost throughout the PWM conversion pulse width. Thus, the PWM conversion circuit 9 outputs PWM signals converted as shown in FIG. 7D.
This makes it impossible to smoothly drive the three-phase coil (not illustrated) of the brushless motor 1 with the pseudo-sinusoidal signals, with the result that the brushless motor 1 rotates with torque fluctuations and with noise. To avoid this, the limiter 5 limits the level of the torque error signal so that the amplitude of the pseudo-sinusoidal signals does not become larger than the predetermined level k. However, the Hall elements 2 (i.e., 2a to 2c) of the brushless motor 1 have large fabrication variations, and this necessitates a complicated setting procedure to set the limit level of the limiter 5 to suit the output characteristics of the Hall elements 2 of the brushless motor 1.
Even when the limit level of the limiter 5 is set optimally, owing to the temperature characteristics of the Hall elements 2, the levels of the Hall signals vary with the ambient temperature, and thus the amplitude of the position signals generated from those Hall signals by the position detection circuit 6 also varies. Since the pseudo-sinusoidal signals are obtained by multiplying the torque error signal, which is limited within the limit level, by those position signals, whose amplitude may thus be too large depending on the ambient temperature, the pseudo-sinusoidal signals can have an amplitude as shown in FIG. 7C, generating PWM signals as shown in FIG. 7D. This causes the brushless motor 1 to rotate with torque fluctuations and with noise. On the other hand, when the amplitude of the position signals becomes too small depending on the ambient temperature, the maximum rotation rate becomes lower than is set, resulting in performance lower than is expected.